Arm A: First line: capecitabine capecitabine 1250 mg/m2 orally b.i.d. on day 1-14 (q3),until progression or unacceptable toxicity. Second line: irinotecan 350 mg/m2 IV infusion on day 1 (q3),until progression or unacceptable toxicity. Third line: oxaliplatin 130 mg/m2 IV infusion on day 1 and capecitabine 1000 mg/m2 orally b.i.d. on day 1-14 (q3). Arm B: First line: irinotecan 250 mg/m2 IV infusion in 30 minutes on day 1 and capecitabine 1000 mg/m2 orally b.i.d. on day 1-14 (q3), until progression or unacceptable toxicity. Second line: oxaliplatin 130 mg/m2 IV on day 1 and capecitabine 1000 mg/m2 orally b.i.d. on day 1-14 (q3), until progression or unacceptable toxicity.